The paper “Local residence-times in continuous flow systems”, P. V. Danckwerts, Chemical Engineering Science, 1958, Vol. 9, pp. 78-79, Pergamon Press Ltd. discusses the theory of local fluid ages and determining an average age in a continuous flow systems as an indication of the degree of stagnation of the material at a point in the system.
Although the patent literature is replete with references to residence time and ageing of reactants there is very little patent literature on methods to achieve appropriate local residence times in a continuous stirred tank reactor.